1. Field of the Invention
The present invention includes a nitrate reduction device for use on environmental monitors. The nitrate reduction device has a flow chamber containing a cadmium structure forming a plurality of longitudinal channels within the flow chamber. The cadmium structure may be formed by a plurality of cadmium containing wires.
2. Brief Description of the Related Art
Nitrite in natural water samples typically is determined colorimetrically using Griess reaction reagents, such as sulfanilamide and N-(1-Naphthyl)ethylenediamine. Equivalent specific and sensitive colorimetric reagents for nitrate determinations are not available. Therefore, nitrate is frequently reduced to nitrite prior to analytical determinations. Reduction of nitrate to nitrite with zinc, hydrazine, bacterially derived nitrate reductase, and ultraviolet radiation has been reported. Additionally, cadmium has been used in packed bed, single wire-in-tube, and open tubular configurations for nitrate reductions.
Zinc provides a problematic reducing agent in that it is overly reactive resulting in reduction of nitrate to other species in addition to nitrite. Reduction of nitrate to nitrite by hydrazine is kinetically unfavorable and difficult to control. Bacterially derived nitrate reductase becomes inactive in air-saturated solutions and requires a hazardous cofactor. Ultraviolet radiation is not kinetically favored and difficult to control. Additionally ultraviolet radiation has power requirements to an ultraviolet lamp that may become prohibitive for remote applications.
For cadmium reduction systems, out gassing of samples and reagent creates void areas in granular cadmium packed bed reactors that lower reactivity. Resistance to flow (back pressure) in packed-bed, granular cadmium reactor systems can also be problematic. Additionally, the dissolution of cadmium granules that occurs as the granules react with nitrate and dissolved oxygen causes progressively increasing void volumes at the head of the reactor, adversely affecting reduction efficiency. Both the single wire-in-tube and the open-tubular reactor are immune to void areas and have lower back pressures, but have lower reactive surface-to-volume characteristics.
Single wire-in-tube cadmium reactors have been disclosed in Stainton, M. P., Anal. Chem., 1974, 46, 1616 (“Stainton Article”); Willis, R. B., Anal. Chem., 1980, 52, 1377–1379 (“Willis Article”); and Willis, R. B. and Gentry, C. E., Commun. In Soil Sci. Anal., 1987, 18, 625–636 (“Willis et al. Article”). The Stainton Article discloses a 1-meter length 1/32-inch i.d. Teflon tubing threaded with a 1-meter length of 1 millimeter diameter (sic) cadmium wire. The Willis Article discloses the use of a wire made of an alloy of 95% cadmium and 5% silver. The Willis et al. Article also discloses the use of a cadmium-silver wire. As such, none of these references address the need for high reactive surface-to-volume characteristics.
For any passive-wall tubular, active-metal, nitrate reduction system of volume, V, with length, L, and diameter, D, the speed and completeness of reduction, referred to as reduction efficiency in discussions that follow, increase as the surface area of the active metal in contact with the nitrate-containing solution increases. In the case of granular-cadmium, packed-bed reactor systems, reduction efficiency increases as the size of cadmium granules decreases. Aforementioned operability problems, including increased back pressure, increases as the size of cadmium granules decrease. In the case of single-wire-in-tube systems, high reduction efficiency is achieved only when the diameter of the cadmium wire approaches the inside diameter of the passive-wall tubular flow chamber. Thus an increase in reactor volume can only be achieved by increasing its length. This characteristic leads to unacceptable amounts sample dispersion (dilution and loss of analytical detection) and back pressure for batch analyzer applications may require reduction systems with liquid volume capacities of several milliliters.
There is a need for high reactive surface-to-volume ratios in cadmium reduction systems, that overcome the problems of low reactivity per unit length of single wire in tube and open tubular cadmium reactors while maintaining advantages of low flow resistance, and that eliminates the void areas and void volume formation problems, including compression, found in granular cadmium packed bed reactors. The present invention addresses this need.